1. Field of the Invention
This invention relates to jet engine testing and more particularly to a calibration system and method used to help analyze inlet flows through a bellmouth test inlet.
2. Description of Related Art
Aircraft gas turbine engines require many ground tests during the development and life time of the engine. Accurate measurement of three "basic" performance parameters, airflow, fuel flow, and thrust, are required to define basic "test-measured" engine performance characteristics. Accuracy requirements for each of the three "basic parameters' are of equal importance to typical subsonic turbofan applications.
For example, at 35,000 feet and Mach=0.8 cruise conditions, a 1% error in engine airflow equates to a 1.25% momentum related error in net thrust and/or SFC for a typical General Electric CF6-8OC2 turbofan engine. The same conditions result in a 1.4% error in net thrust and/or SFC for a General Electric CFM56-3B turbofan engine.
It is very important to be able to accurately measure the basic performance parameters with calibration verifiable systems. All test procedures have fundamentally inherent uncertainties that translate into uncertainties in both test measured and flight guaranteed performance capabilities of the engine. Therefore it is very important to get as accurate measurements as possible.
Industry accepted standards and equipment needed to calibrate both thrust and fuel flow measuring systems have been available for many years. Thrust measurement systems capable of .+-.0.1% accuracy levels, and fuel flow measurement systems capable of .+-.0.5% accuracy levels, based on National Bureau of Standards (NBS) traceable calibration standards, are in current use in the industry. Comparable degrees of airflow measurement accuracy are not yet available. There are no comparable standards, methods, and/or equipment capable of calibrating current and new large turbofan air metering bellmouths.
Furthermore, the airflow levels of newer engine models having higher thrust levels and airflow, up to 2000 lbs/sec for the General Electric CF6-80E and up to 3200 lbs/sec for the General Electric GE90, far exceed current measurement system capabilities of most of the large engine altitude test facilities (AEDC, NGTE, and P&WA Wilgoose). The problem is particularly acute for large fan engines because of the large size of the bellmouths. The problem is further heightened because some type of back-to-back test program to compare test measured engine airflow data obtained from bellmouth systems to data obtained from some other type of system, i.e. having venturi based equipment, in another facility is not feasible.
One conventional type of calibration of large engine bellmouths has been accomplished by a procedure which employs metering plane stream static pressure measurements obtained from currently available pitot-static inlet rakes. The rake measured stream static pressure measurements, used to determine local velocity levels and distributions, are compared to predicted levels which are based on "bellmouth measured," and therefore allegedly validated, wall static referenced flow rates. The system is somewhat flawed and limited by the relatively large inherent uncertainties in the rake measured stream static pressure levels, and the fixed number of sample locations, typically six immersions and four rakes, which provide representative, but incomplete measurements of the wall-to-wall distribution of stream flow characteristics.
The uncertainties inherent in the rake measured stream static pressure levels are due to flow field changes caused by the rake and more particularly related to the rake size, shape (which causes blockage) characteristics, and the bellmouth size, shape, and flow field characteristics. Pylon shaped rakes have different pressure coefficients at each immersion because of strut chord and thickness related influence on the measurement location pressure levels. The same rake set installed in two different bellmouths, may have two significantly different, installation peculiar, sets of pressure coefficients. Rake probe pressure coefficient levels are peculiar to the rake biased bellmouth flow field characteristics and, as such, cannot be accurately evaluated in some other, significantly different, flow field environment. Conventionally, rakes are calibrated at locations other than in situ as in the bellmouth and therefore are subject to various anomalies which reduce the accuracy of the rake type pressure measuring device.
Other considerations include the length of the bellmouth and the length of the pressure probes. An ideal bellmouth would be long enough to completely smooth out and straighten the flow within, but practical weight and size requirements do not allow for such ideal conditions. The pressure probes are relatively short and flow around the static probe aperture is influenced by both the nose of the probe and the rake body itself which adds uncertainty to the measurements. The probes cannot be lengthened for they would be subject to failure inducing vibrational loads.
Therefore a primary objective of the present invention is to provide an accurate system of calibrating a rake mounted stream static pressure measuring device used for determining airflow velocities in a bellmouth inlet of a turbofan engine test apparatus.